Saurian Malarial Parasites in Eastern Panama

SYNOPSIS Plasmodium gonatodi sp. nov. is described from Gonatodes albogularis fuscus of eastern Panama. It is characterized by elongate gametocytes and polymorphic schizonts containing 12-46 nuclei when apparently mature. Both proerythrocytes and erythrocytes are commonly parasitized, host cells are hypertrophied and distorted, and their nuclei are displaced. Prematuration sexual stages may be irregularly shaped and larger than mature gametocytes.
Plasmodium diploglossi Aragão and Neiva , 1909 is reported from Mabuya mabouya in eastern Panama, and Plasmodium morulum sp. nov. is described from this host. P. morulum usually parasitizes immature erythrocytes, and is characterized by lenticular or oval to round gametocytes, and schizonts with 14-40 nuclei usually arranged in a globular mass. Host cells are slightly hypertrophied and distorted, and their nuclei are usually displaced. Inoculation of infected blood into clean hosts produces numerous schizonts in white cells as well as in the erythrocyte series.
Pigment in both P. gonatodi and P. morulum , if present, consists of a few minute dark dots which do not meet the polarized light test for hemozoin.     

A New Saurian Malaria Parasite Plasmodium clelandi sp. n. from Ceylon*

Plasmodium clelandi sp. n. is described from Varanus cepedianus of Mannar District, Ceylon. The earliest asexual stages were seen as a chromatin dot. Trophozoites have a vacuole with chromatin material spread around its periphery. The trophozoites divide and mature to form the schizont. The maximum number of merozoites observed in a schizont was 8. Pigment was scanty and sometimes absent. The gametocytes had a characteristic elongate form and tend to encircle the host cell nucleus.     

Receptors for the Malarial Parasite

Abstract

During the erythrocytic cycle of Plasmodium, the parasite develops within an enclosed space, the parasitophorous vacuole, formed by endocytosis of an invasive stage, the merozoite. Among the erythrocyte membrane proteins possibly acting as a receptor for the attachment of P. falciparum merozoites to human erythrocytes is glycophorin A Isolated glycophorin inhibits merozoite entry in a competitive manner, perhaps via association with a 155 kDa surface protein. Another protein that competitively inhibits merozoite invasion, is band 3, the erythrocyte anion transport protein. The protein bearing Duffy blood group antigens may act to modulate invasion, but does not behave as a receptor.     

Effects of Chloroquine on the Feeding Mechanism of the Intraerythrocytic Human Malarial Parasite Plasmodium falciparum1

Ultrastructural investigations of P. falciparum cultivated in vitro in human erythrocytes revealed new features of the feeding mechanism of the parasite. Mature trophozoites and schizonts take up a portion of the host cytosol by endocytosis which is restricted to cytostomes and which involves the invagination of both parasitophorous and parasite membranes. The resulting endocytic vesicles, surrounded by two concentric membranes, migrate towards the central food vacuole membrane. The external membrane of the endocytic vesicles apposes that of the food vacuole, leading to the internalization of vesicles bounded by a single membrane into the vacuolar space where they are rapidly degraded. We conclude from this sequence of events that endocytic vesicles fuse with the food vacuole. Treatment of infected cells with therapeutic concentrations of chloroquine inhibited the last step of the feeding process, i.e. vacuolar degradation. This was manifested by the accumulation within the vacuolar space of intact vesicles bounded by single membranes. The implications of these findings for the antimalarial activity of chloroquine are discussed.     

An Electron Microscopical Study of the Interaction of Monoclonal Antibodies with Gametes of the Malarial Parasite Plasmodium gallinaceum1

Anti-malarial gamete antibodies prevent the fertilization of gametes in the mosquito midgut and prevent transmission of malaria. Recently, hybridoma cell lines secreting monoclonal antibodies (10G3 and 11C7) against gametes of the malarial parasite have been developed. These antibodies act synergistically to mediate 80–90% suppression of the infectivity of gametocytes, although neither monoclonal antibody alone has a significant effect on gametocyte infectivity. We performed immuno-electron microscopy to characterize the interactions of these monoclonal antibodies with gametes of Plasmodium gallinaceum. Male gametes exposed to either 10G3 or 11C7 agglutinated into loose clusters, while those exposed to a mixture of 10G3 and 11C7 agglutinated into long, rope-like bundles. This difference appears to be related to the distribution of the antibodies on the surface of the gametes. When 10G3 or 11C7 labeled with a ferritin-conjugated anti-mouse Ig were used singly, the ferritin particles were distributed in focal areas over the surface of the parasites. By contrast, when the male gametes were exposed to a mixture of 10G3 and 11C7, the ferritin particles were distributed over their entire surface. Female gametes reacted similarly to these antibodies. These observations indicate that combinations of antibody specificities that reduce fertilization efficiency coat the entire surface of the gametes. On the other hand, focal interactions resulting from a single antibody are unable to block fertilization.     

Coenzyme A and the Malaria Parasite Plasmodium lophurae

The extracellular development of Plasmodium lophurae in vitro was favored by the addition, to the erythrocyte-extract medium, of a coenzyme A (CoA) preparation of about 75% purity. The effect of CoA was the same regardless of the concentration of free pantothenate in the medium, indicating that the parasites require the complete coenzyme rather than its pantothenic acid moiety. Erythrocyte extracts were found to contain enzymes which hydrolyzed added CoA at a rate such that 8 units per ml. of coenzyme was only slightly destroyed after 3 hours'incubation but almost completely destroyed after 18 hours'incubation at 40°C. The CoA content of erythrocytes from chickens or ducks heavily infected with P. lophurae was about twice as high as that of erythrocytes from uninfected birds. The increased CoA was associated with the parasites, an observation suggesting that malaria parasites can accumulate this essential growth factor which they cannot synthesize. The CoA concentration in the livers of infected chickens was approximately 40% lower than that in the livers of control chickens. The livers of ducks on the 6th day of infection had a slightly lower CoA concentration than those of uninfected ducks. This depletion in CoA, together with the depletion in biotin previously demonstrated in P. lophurae infection, may play a role in the pathology of this infection.     

A New Zamia Species from the Panama Canal Area

There are four terrestrial, above-ground stemmed Zamia taxa in Panama, the species delimitations of which have been a matter of controversy or misplacement at one time or the other (Schutzman et. al., 1998; Stevenson, 1993; Taylor, 1999a, b, 2002). All are allopatrically distributed. Two are in western Panama in Chiriquí province. One of these, Z. pseudomonticola, is found in the northwest of the province at altitudes above 1000 m, while the other, Z. fairchildiana, is found in a relatively small patch of forest in southwestern Chiriquí province. The other two taxa are found around the Canal area or farther east. The species described in this paper is found near the Canal area, and the last of the group, Z. elegantissima, occurs north of the Canal area in the province of Colon and also some distance to the east, including part of the Dule or Kuna aboriginal homeland known as Kuna Yala. After 16 years of research on the new taxon, we have decided to describe it as a new species, pointing out its similarity to Z. elegantissima and its distinctness from Z. pseudomonticola and Z . fairchildiana. The two western species are more alike in structure compared with the eastern species, and the latter are more alike between themselves compared to the western taxa. Even so, there are differences in vegetative and reproductive structures to clearly separate each species. There are even differences in the pollinators, these being, in all cases found, species of the weevil genus Rhopalotria and the snubbed-nosed beetle Pharaxonotha.     

Phagotrophy and Two New Structures in the Malaria Parasite Plasmodium berghei

Blood collected from rats infected with Plasmodium berghei was centrifuged and the pellet was fixed for 1 hour in 1 per cent buffered OsO₄ with 4.9 per cent sucrose. The material was embedded in n-butyl methacrylate and the resulting blocks sectioned for electron microscopy. The parasites were found to contain, in almost all sections, oval bodies of the same density and structure as the host cytoplasm. Continuity between these bodies and the host cytoplasm was found in a number of electron micrographs, showing that the bodies are formed by invagination of the double plasma membrane of the parasite. In this way the host cell is incorporated by phagotrophy into food vacuoles within the parasite. Hematin, the residue of hemoglobin digestion, was never observed inside the food vacuole but in small vesicles lying around it and sometimes connected with it. The vesicles are pinched off from the food vacuole proper and are the site of hemoglobin digestion. The active double limiting membrane is responsible not only for the formation of food vacuoles but also for the presence of two new structures. One is composed of two to six concentric double wavy membranes originating from the plasma membrane. Since no typical mitochondria were found in P. berghei, it is assumed that the concentric structure performs mitochondrial functions. The other structure appears as a sausage-shaped vacuole surrounded by two membranes of the same thickness, density, and spacing as the limiting membrane of the body. The cytoplasm of the parasite is rich in vesicles of endoplasmic reticulum and Palade's small particles. Its nucleus is of low density and encased in a double membrane. The host cells (reticulocytes) have mitochondria with numerous cristae mitochondriales. In many infected and intact reticulocytes ferritin was found in vacuoles, mitochondria, canaliculi, or scattered in the cytoplasm.     

Poisoning Pyridoxal 5-Phosphate-Dependent Enzymes: A New Strategy to Target the Malaria Parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is able to synthesize de novo pyridoxal 5-phosphate (PLP), a crucial cofactor, during erythrocytic schizogony. However, the parasite possesses additionally a pyridoxine/pyridoxal kinase (PdxK) to activate B6 vitamers salvaged from the host. We describe a strategy whereby synthetic pyridoxyl-amino acid adducts are channelled into the parasite. Trapped upon phosphorylation by the plasmodial PdxK, these compounds block PLP-dependent enzymes and thus impair the growth of P. falciparum. The novel compound PT3, a cyclic pyridoxyl-tryptophan methyl ester, inhibited the proliferation of Plasmodium very efficiently (IC₅₀-value of 14 µM) without harming human cells. The non-cyclic pyridoxyl-tryptophan methyl ester PT5 and the pyridoxyl-histidine methyl ester PHME were at least one order of magnitude less effective or completely ineffective in the case of the latter. Modeling in silico indicates that the phosphorylated forms of PT3 and PT5 fit well into the PLP-binding site of plasmodial ornithine decarboxylase (PfODC), the key enzyme of polyamine synthesis, consistent with the ability to abolish ODC activity in vitro. Furthermore, the antiplasmodial effect of PT3 is directly linked to the capability of Plasmodium to trap this pyridoxyl analog, as shown by an increased sensitivity of parasites overexpressing PfPdxK in their cytosol, as visualized by GFP fluorescence.